A solar cell converts solar energy directly to DC electric energy. Generally configured as a photodiode, a solar cell permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier, such as an electron or a hole (a lack of an electron), may be extracted as current.
Most solar cells are generally formed on a silicon substrate doped with a first dopant (often boron) forming an absorber region, upon which a second counter dopant (often phosphorous), is diffused forming the emitter region, in order to complete the p-n junction. After the addition of passivation and antireflection coatings, metal contacts (fingers and busbar on the emitter and pads on the back of the absorber) may be added in order to extract generated charge.
In general, a low dopant atom concentration within an emitter region will beneficially result in low recombination (and thus higher solar cell efficiencies), but detrimentally result in poor electrical contact (high resistance) to metal electrodes. Conversely, a high dopant atom concentration will detrimentally result in high recombination, but beneficially result in low resistance ohmic contacts to metal electrodes. This is particularly true when the front metal electrode is formed using a screen printed silver paste. Often, in order to reduce manufacturing costs, a single uniform dopant diffusion is used to form an emitter. However, the doping concentration is generally a compromise between low recombination and a good metal contact. That is, low recombination can generally be achieved at lower doping, however higher doping is required for contact formation. Consequently, the resulting solar cell efficiency (the percentage of sunlight that is converted to electricity) is generally reduced as a result of such a compromise.
Referring now to FIG. 1, a simplified diagram of a traditional front-contact solar cell is shown. In a common configuration, an n-type diffused region 108 is first formed on a p-type (lightly doped) silicon substrate 110. Prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, residual PSG formed on the substrate surface during the phosphorous deposition process (i.e., POCl3) is substantially removed, commonly by exposing the silicon substrate to hydrofluoric acid (HF). The set of metal contacts, comprising front-metal contact 102 and back surface field (BSF)/back metal contact 116, are then sequentially formed on and subsequently fired into silicon substrate 110.
The front metal contact 102 is commonly formed by depositing an Ag (silver) paste, comprising Ag powder (about 70 to about 80 wt % (weight percent)), lead borosilicate glass (frit) PbO—B2O3—SiO2 (about 1 to about 10 wt %), and organic components (about 15 to about 30 wt %). After deposition the paste is dried at a low temperature to remove organic solvents and fired at high temperatures to form the conductive metal layer and to enable the silicon-metal contact. During the firing process, as the temperature is increased up to about 400° C., the frit softens and forms a molten glass which wets and dissolves the underlying anti-reflective coating (e.g., silicon nitride) barrier layer 106 layer in an exothermal redox reaction:2PbOGlass+Si2Pb+SiO2Glass   [Equation 1]
In view of the foregoing, there is a desire to provide method of forming a low resistance silicon metal contact on the low-doped emitters.